1. Field of the Invention
The present invention relates to a photo thin film transistor, and more particularly, to a photo thin film transistor having photoconductive characteristics and a unit cell of an image sensor using the same.
2. Description of the Related Art
Generally, the development of information and communication technology has led to technical development such as increases in processing speed and capacity storage. Devices used for information storage include optical information storage devices such as compact discs (CDs) and digital versatile discs (DVDs), and electric memory devices such as dynamic random access memory (DRAM). Examples of devices used in the field of information storage and processing include a photo thin film transistor and a complementary metal oxide semiconductor (CMOS) image sensor. A thin film transistor is generally fabricated using a CMOS process.
FIG. 1 is a cross-sectional view of a photo thin film transistor fabricated using a general CMOS process.
Referring to FIG. 1, an amorphous silicon layer 105 is formed on a silicon substrate 100 doped with impurities. To obtain an ohmic contact, source and drain ohmic contact portions 115 and 110 are formed on respective sides of the amorphous silicon layer 105. The source and drain ohmic contact portions 115 and 110 are formed using an ion implantation process of implanting impurities into part of the amorphous silicon layer 105. The source and drain ohmic contact portions 115 and 110 connect a source electrode 125 and a drain electrode 120 respectively. A gate insulating layer 130 is formed on the amorphous silicon layer 105, the source and drain ohmic contact portions 115 and 110, and the source and drain electrodes 125 and 120. The gate insulating layer 130 is an oxide layer. A gate electrode 135 is formed of metal and disposed on the gate insulating layer 130.
However, the performance of the photo thin film transistor of FIG. 1 is not good because the amorphous silicon layer 105 has low photoconductivity.
Further, when fabricating the photo thin film transistor of FIG. 1 using the CMOS process, a high process temperature of about 500° C. to 1000° C. is required. Furthermore, the photo thin film transistor of FIG. 1 necessarily requires a high-priced silicon substrate and ion implantation process. Thus, the cost of fabricating the thin film transistor of FIG. 1 using the CMOS process is very high.